The present invention relates to a signal-processing circuit of a heat-sensitive recording apparatus in which a heat-generating member is heated by being energized, and a recording medium such as inking paper provided in contact with the heat-generating member is heated for transfer to the recording paper by sublimation or fusion, thereby reproducing an image on the recording paper, or more in particular to a signal-processing circuit for reproducing an image with a gradation by controlling the energization time of the heat-generating member.
Generally, the relationship between the time of heat application to a recording medium such as inking paper, that is, the energization time of a heat-generating member and the pigment concentration (coloring concentration) on the recording paper is not linear as shown in FIG. 5. As apparent from the energization time-coloring concentration characteristic shown in FIG. 5, the inclination is gentle at the high-concentration and low-concentration parts, requiring a longer energization time for obtaining the same concentration change.
If an original image is to be correctly reproduced from a video signal of an image having gradations (a plurality of levels of the coloring concentration), therefore, it is necessary to change the energization time of the heat-generating member in accordance with the gradation.
Conventional heat-sensitive recording apparatuses comprise a read-only memory (called a ".gamma. memory") for storing the energization time-coloring concentration characteristic (called a ".gamma. characteristic"), whereby at the time of printing, the video signal is converted to generate a train of energization codes representing the energization time of the heat-generating member corresponding to the video signal from the data read from the .gamma. memory, and the heat-generating member is energized by supplying current thereto for a time period corresponding to the data of the energization code train thereby to produce a print. A heat-sensitive recording apparatus of this type is disclosed in Japanese Examined Patent Publication No. 59-17954 and Japanese Unexamined Patent Publication No. 57-48868.
The .gamma. characteristic depends on the ambient temperature of the heat-generating member.
FIG. 6 shows the .gamma. characteristic for two different ambient temperatures. The .gamma. characteristic of FIG. 6(b) represents an ambient temperature higher than that of FIG. 6(a).
Because of this dependency of the .gamma. characteristic on the ambient temperature of the heat-generating member, if a print of a gradation equal to that of an original image is to be obtained, it is necessary to store data on the .gamma. characteristics for a plurality of ambient temperatures in memory and switch the data on the .gamma. characteristics in accordance with the actual ambient temperature, resulting in a bulky memory.
For producing a color print, on the other hand, it is necessary to lay prints of three colors one on another with inking paper of the three primary colors. The inking paper of the three primary colors have different .gamma. characteristics.
FIG. 7 snows example of the .gamma. characteristics of the three primary colors of cyan (Cy), magenta (Mg) and yellow (Ye). in this case, the inclination of the .gamma. characteristic decreases for cyan (Cy), magenta (Mg) and yellow (Ye) in that order.
Production of a color print, therefore, requires the .gamma. characteristics for each of the three primary colors to be stored in the .gamma. memory, thereby further increasing the capacity of the .gamma. memory.
If about ten different characteristic values at 5.degree. C. intervals of the temperature near the heat-generating member are set in the ROM in advance for the recording media representing the three colors, and the characteristic to be read out is changed each time of printing, then the required capacity of the ROM becomes 30 times larger. If the number of gradation is to be increased twice, on the other hand, the required ROM capacity is as much as 60 times larger, thereby making practical applications difficult.